Bond energies of weakly bound molecules

The rate of reaction of elementary bimolecular reactions in the gas phase is usually determined by the height of the energy barrier between reagents and products. Most reactions proceed exponentially faster as the temperature is raised according to the Arrhenius rate law. This is because the fraction of molecules with sufficient energy to surmount the barrier is determined by the Boltzmann distribution function. Some reactions however exhibit a negative temperature dependence in which the rate of reaction increases as the temperature decreases. This so-called non Arrhenius behaviour occurs when the reaction is barrierless. An interesting class of reactions show Arrhenius behaviour at high temperatures but also speed up at low temperatures. This behaviour is attributed to the existence of long range weakly bound complexes in the entrance channel due to van der Waals forces between the reagents, and an important class of reactions which often exhibit this behaviour are those between radicals and molecules. These reactions are of particular importance in the cold environments of the tropopause of the earth's atmosphere, the atmospheres of the gaseous planets and some of their moons, and in the gas clouds surrounding young stars in interstellar space. Recently it has become possible to trap these weakly bound species in the laboratory and to study their properties. An experiment carried out at the University of Pennsylvania by Marsha Lester and her co-workers measures the infrared absorption spectra of these complexes by exciting them with a pulse of infrared radiation and observing one of the resulting fragments as the complex is heated using laser induced fluorescence. By calculating the energy difference between the energy of the dissociating photon and the maximum internal energy observed in the photofragment they are able to deduce an upper bound for the bond dissociation energy of the complex. In the case of the hydrotrioxy radical, HO-OO, the value they so obtain is large enough that in the upper earth atmosphere about a quarter of the OH radicals would be expected to be complexed with molecular oxygen and if true would provoke a significant rethink of our understanding of the chemistry of the tropopause. Theory on the other hand predicts a bond dissociation energy for HO-OO that is about half the value suggested by the experiment. The reason for the discrepancy is most probably due to the assumption that the dissociation is barrierless and what is required is a direct measurement of the bond energy of the complex. We propose to do this using a technique called velocity map imaging. Even though we are fairly confident that we understand the reason for the apparent discrepency between theory and experiment in the case of the hydrotrioxy radical, our experiment would be the first direct measurement of the dissociation energy and would provide the essential data needed to assess the true atmospheric importance of the species. More generally we propose to study a number of other similar complexes and obtain accurate data on their dissociation dynamics by measuring properties such as the internal energy distribution in the photoproducts and correlations between their recoil velocity vectors and rotational angular momenta. The principal output of our research will be to provide data on the bond energies of these complexes which may be compared to quantum mechanical electronic structure calculations and fed into chemical kinetic models of the reactions of radicals and molecules.

The most obvious tangible output of this research will be data on the bond dissociation energies (BDE) of weakly bound complexes of the hydroxyl radical with molecules such as oxygen, sulphur dioxide and acetylene. These data are required in order to assess the importance of the van der Waals potential energy minimum that exists in the entrance channel of the reaction coordinate and which can lead to markedly non-Arrhenius behaviour of the rate law. Reactions of the OH radical are particularly important in the terrestrial atmosphere as OH is the principal oxygenating species and consequently our measurements are expected to have a significant impact on chemical models of atmospheric chemistry. In the case of OH plus oxygen it has been suggested that the well depth in the entrance channel may be as large at 22 kJ/mol. If this were true then current models predict that up to 25% of OH radicals in the vicinity of the tropopause would be associated with oxygen as the weakly bound hydrotrioxy radical, and this would necessitate a complete reappraisal of our model of the atmosphere. However, the current experimental determination of the strength of the HO-OO bond energy is only an upper bound deduced by conservation of energy arguments which assume that there is no barrier along the dissociation coordinate. Our proposed experiment will provide the first direct measurement of the BDE. The existence of van der Waals minima in the entrance channels to reactions with small but significant barriers is also likely to impact on current models of astrochemistry and the chemistry of the outer planets and their moons. Up to now these reactions have usually been discounted from the models because in the cold environment of interstellar space there is not sufficient energy to overcome the barrier; however if the reagents become trapped in the entrance channel well this then gives enough time for quantum mechanical tunnelling to play an important role; the reaction of OH and acetylene is such an example. Although in our proposal we focus on the reactions of the hydroxyl radical our methodology can be easily extended to other radical species such as CN. We propose to collaborate closely with the kinetic modelling groups in the department of Physics and Astronomy, the Institute for Climate and Atmospheric Science and the School of Chemistry at Leeds in order to assess the importance of weakly bound species on planetary atmospheres and interstellar chemistry. In order to connect our experimental data with kinetic measurements carried out in Laval nozzles (CRESU experiments) and cryogenically cooled reaction vessels we will also collaborate closely with ab initio quantum chemists (we have in mind the group of Fred Manby in Bristol) to obtain accurate potential energy surfaces on which we can run classical trajectory and phase space calculations in collaboration with our theoretical colleagues in Leeds (Dmitrii Shalashilin). The techniques we will develop in the course of the research programme; intense molecular beam sources of radicals, vacuum ultraviolet light sources, imaging of slowly moving photofragments etc., also directly impact on the research programmes of our collaborators within an EU ITN (ICONIC) and this will ensure rapid dissemination across Europe and the rest of the world of the results of our research. Some funds are requested for attendance at multidisciplinary international conferences in order to assist in disseminating the results to an audience outside the spectroscopy and dynamics community.